Titanium is a lightweight material whose specific gravity is as low as about half that of steel and which is characterized by its high corrosion resistance and high strength. Titanium is therefore used for parts of aircrafts, railway vehicles, two-wheeled vehicles, automobiles, etc. for which reduction in weight is greatly desired, home appliances, members for construction, etc. Titanium is also used as a material for medical use because of its high corrosion resistance.
However, applications of titanium are limited due to its high material cost, as compared to iron and steel materials and aluminum alloys. In particular, titanium alloys have tensile strength as high as more than 1,000 MPa, but do not have enough ductility (elongation at break). Moreover, titanium alloys have poor plastic workability at normal temperature or in a low temperature range. Pure titanium has elongation at break as high as more than 25% at normal temperature and has excellent plastic workability in a low temperature range. However, pure titanium has tensile strength as low as about 400 to 600 MPa.
Various studies have been carried out in response to a very strong need for titanium having both high strength and high ductility and for reduction in material cost of titanium. In particular, many techniques of strengthening titanium by using relatively inexpensive elements such as oxygen rather than expensive elements such as vanadium, scandium, and niobium have been studied as related art in order to achieve cost reduction.
For example, Japanese Unexamined Patent Application Publication No. 2012-241241 (Patent Literature 1) proposes the following steps as a method for producing a solid solution of oxygen in titanium material.
(a) preparing titanium powder and TiO2 particles;
(b) mixing the titanium powder and the TiO2 particles so that the mixture contains 0.5 to 3.0 mass % of TiO2 particles; and
(c) sintering the mixture in a vacuum atmosphere in the temperature range from 700° C. to a temperature lower than the melting point of TiO2 to thermally decompose the TiO2 particles so that oxygen atoms dissociated form a solid solution with titanium.